1. Field of the Invention
The present invention concerns improvements in or relating to the demodulation of frequency shift keyed (FSK) optical transmissions.
Coherent optical communication systems offer higher practical receiver sensitivity and higher wavelength selectivity than direct detection systems. These advantages are gained at the expense of severe stability requirements on the local oscillator and signal lasers. Estimates suggest that the combined local oscillator and transmitter laser linewidth must be less than 0.5% of the bit rate for PSK systems, although this can be relaxed to 10-30% for asynchronous ASK and FSK heterodyne systems. Frequency shift keying (FSK) is attractive from this standpoint and also because it allows direct modulation of the transmitter injection laser via the drive current. Hitherto major drawback of FSK systems has been the need for receivers having four times the bandwidth of the baseband data.
2. Description of Related Art
In direct FSK demodulation, optical frequency modulated signals are demodulated using an optical frequency discriminator. Though direct detection is a basic and simple configuration for optical FSK systems, it does not bring about any improvements in the received signal power level.
In a conventional FSK heterodyne system (FIG. 1) optical signals from the FSK transmitter are demodulated in two steps of optical and electrical demodulation. The transmitted beam is coupled with a beam from a local oscillator 3 and is converted into an IF signal by means of a square-law detector 5. This produces an electrical IF signal which then is amplified and filtered at 7. It is then demodulated to the baseband signal by an RF frequency discriminator 9 and the IF signal passed via a baseband filter 11. IF signal frequency stability is a requisite for stable FSK signal demodulation. It is usual therefore to include a feedback loop amplifier 13, following the discriminator 9, to control the frequency of the local oscillator laser 3. A bandwidth for the receiver amplifier, of four times that of the baseband data is usual for this system.
An alternative approach to this is to use a single filter envelope detector having a cut-off midway between the upper and lower signal tones. See Saito et al, "S/N and error rate evaluation for an optical FSK heterodyne system using semiconductor lasers", IEEE Journal of Quantum Electron. 1983 QE-19 pp 180-193. This then has the advantage of a reduced bandwidth, but this is at the expense of a 3 dB loss in signal sensitivity.